I print my own w/The Print Shop. I also take ads out of magazines, train books, etc.
With Print Shop software you can do almost anything, including making your own decals, teeshirt iron-ons, stickers, etc.
I make my own RR business cards w/Print Shop.
As far as downloading anything off the internet--There is a web site that lets you do that for about $29.95

http://www.trains.com/Content/Dynamic/Articles/000/000/000/933naqaa.asp (Article on this topic)

http://www.trains.com/community/forum/topic.asp?TOPIC_ID=4463 (My posting on how to fit signs to your buildings' windows, and more links).

Some people warn about copyright stuff. As long as you are not selling these ads, and just use them for your personal layout, there's no worry. What's the difference between this, and hanging up an old metal "Sinclair Oil" or beer sign in your garage?